MBB 323

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MBB 323

• Lecture 2
• Office hours Friday 230

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Defining a System Boundaries

• A piece of the universe.
• ranging from galaxies to molecules.
• Normally defined by a boundary.
• Boundary can be real or imaginary.
• Boundaries can move (volume changes).
• Specifies an inside from the surroundings.
• A useful booking keeping device.

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An unstable system
Economist, Sept. 5, 03
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Thermodynamic systems are defined by their
boundary properties

• System Type Boundary Permits flow of
• Open Energy, volume matter.
• Closed Energy volume.
• Adiabatic No heat energy.
• Isolated Nothing in or out.

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The first law of thermodynamics Conservation of
energy

• Energy can be transferred across a system
  boundary, but the total energy found in the
  system and the surroundings cannot change.
• Energy flowing into a system is positive.
• Energy flowing out of a system is negative.

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Forms of Energy

• Kinetic energy
• Translational and rotational motion of a
  particle.
• Momentum and pressure at a boundary.
• Related to heat energy (more later).
• Potential energy
• Requires an interactive force and depends on
  relative positions.
• Gravitational energy, electrostatic energy
• Hookes law (p 18 of text).
• Radiant energy
• Photons contain energy.

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Important theoriesthat fit within the tenants of
thermodynamics

• QED Quantum electrodynamics
• Describes all of chemistry and much of physics
• Basic particles in the theory are
• The electron (negatively charged)
• The photon
• Theory explains how electrons interact with
  charged particles through photon mediated
  interactions.
• Does not explain
• Nuclear processes (not cellular!)
• Gravitation

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An ancient water pump
V (m/s)
The pump system
To field
Aout (m2)
h (m)
V (m/s)
From valley stream
Ain (m2)
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Volume in must equal volume out(or pump will
explode)
V (m/s)
The pump system
To field
Aout (m2)
h (m)
V (m/s)
V (m/s)
From valley stream
Ain - Aout (m2)
Ain (m2)
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Energy flow terms

• Kinetic energy in per second (power)
• If M kg/s passes through the pipe then
• Kin 1/2MV21/2(rVAin)V21/2rAinV3
• Now as the two outflow pipes have areas that add
  up to the input and the velocities are postulated
  to be the same, we must have that
• Koutflows Kin

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Better, but energy was not conserved the water
in the field has potential energy.
V (m/s)
The pump system
To field
Aout (m2)
hout (m)
V (m/s)
V (m/s)
From valley stream
Ain - Aout (m2)
Ain (m2)
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Energy flow terms

• As the kinetic energies flows in and out are
  balanced Koutflows Kin
• The gravitational work done to lift the water to
  the field must be balanced by something else.
• One way to do this is to lower the waste pipe
  (how else?).

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If we lower the exhaust pipe we can balance out
the gravitational energies.
V (m/s)
The pump system
To field
Aout (m2)
hout (m)
V (m/s)
V (m/s)
From valley stream
hdrop (m)
Ain (m2)
Ain - Aout (m2)
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Potential Energy flow terms

• If M kg of water are lifted per second, then the
  pump system must expend the following amount of
  power
• Pout-(rVAout)ghout (minus as lost to field)
• The power gained by lowering the waste water
  is
• PdroprV(Ain-Aout)ghdrop (plus as gained by pump)

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Conserve Energy

• Total energy in has to equal energy out, if no
  energy can accumulate in the pump, so
• KinKoutflowsPoutPdrop
• -PoutPdrop
• rVAoutghout rV(Ain-Aout)ghdrop
• (Ain-Aout)/ Aout hout /hdrop

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Farmer can pump a fraction of the water to the
field
V (m/s)
The pump system
To field
Aout (m2)
hout (m)
V (m/s)
V (m/s)
From valley stream
hdrop (m)
Ain (m2)
Ain - Aout (m2)
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Could you design a pump that did this?

• Electric generator, turned by falling water,
  drives electric pump

• Actual pump normally is much more primitive Uses
  two valves.

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Summary

• A system is defined by a boundary.

• The total energy change between a system and its
  surroundings does not change.
• Energy can take a variety of forms (read more in
  your text).